I use the standard fly with my hammock and I've been through some pretty heavy rain and wind storms without a drop of water.
http://www.bicycletouring101.com/HammockNoTrees.htm has a write-up of my experience using a Hennessy Hammock. The second picture on the right hand side of the page actually shows an exposed position where I went through a storm with winds recorded at 80 km/hr (50 miles/hour).
I take a thermarest with me even through I own the under and over pads. With the thermarest I can set up the hammock in bivy mode and have a reasonably good sleep if trees are unavailable. (
http://www.bicycletouring101.com/HammockNoTrees.htm )
Hammocks of any kind do have the risk of a tree coming down on you when you go to get into the hammock if you haven't carefully checked out the conditions of the trees that you connect to.
(
http://www.bicycletouring101.com/HammockDanger.htm )
I also own the hex fly. It makes a wonderful bike cover for stealth camping. Despite the tons of reflective material on my bike and saddlebags once the cover is over top the bike disappears.
~Jamie N